One generally known industrial burner model, which is intended for gaseous and/or liquid fuels, comprises in connection with the windbox a fuel supply conduit for the combustion head and opening to the combustion head, as well as a combustion air supply conduit opening into the windbox. The windbox is associated with a combustion chamber, such as a boiler, which opens into a flue gas conduit. The burner operation is controlled by burner automation, comprising measuring instruments which include in particular a lambda sensor that measures the amount of residual oxygen in flue gases.
In another generally known, so-called monoblock industrial burner, intended for gaseous and/or liquid fuels, the air is supplied directly to the combustion head of a burner with an air blower included in the burner.
When an industrial burner is used for the combustion of liquid or gaseous fuels, there is a problem that the thermal combustion process always generates nitrogen oxides (NOx) because, at a high temperature (>1000° C.), the atmospheric nitrogen or organic nitrogen contained in fuel reacts with combustion air or oxygen contained in fuel. The higher the temperature and the longer the burn time, the more NOx emissions are produced. Another problem in the thermal combustion process is that thermal combustion is never complete, but the flue gas is always left with unburned hydrocarbons (VOC) and carbon monoxide (CO) as a result of incomplete combustion. The resulting amount of these is the higher, the lower the temperature and the shorter the burn time.
Therefore, the emissions resulting from reducible (NOx) and oxidizable (HC and CO) reactions are generated in conflicting temperature conditions, hindering the reduction thereof. Authorities have started to introduce stricter emission regulations based i.a. on BAT (Best Available Technology) resolutions in Europe BAC (Best Available Control) standards in the USA.
One possibility of reducing the amount of emissions is to use catalytic post-combustion known from the Applicant's WO application No. 2014/154931 in connection with the above-described burner by placing a catalytic converter in a combustion chamber, such as a boiler or flue gas conduit, present in association with the burner. In one embodiment of the above-mentioned patent application, the fuel is pre-combusted partially in at least one thermal pre-combustion zone of the burner and thereafter the post-combustion of pre-combustion-generated gases is carried out in at least one post-combustion zone provided with a catalytic converter for burning the pre-combustion-generated gases, for the reduction of pre-combustion-generated NOx's, and/or for the oxidation of hydrocarbon and carbon monoxide emissions. The post-combustion is conducted in at least one catalytic zone. In one embodiment of the above-mentioned application, the apparatus comprises a thermal burner which is supplied with a liquid or gaseous fuel, and the apparatus is further provided with at least one catalytic converter for the reduction of NOx's present in flue gases generated in thermal combustion, as well as for the oxidation of hydrocarbon and carbon monoxide emissions.
In case the aforesaid burner is adjusted without a feedback from flue gases, the oxygen content of flue gases shall vary roughly +/−1%, which corresponds to about 10% of the amount of combustion air. In a non-feedback system, 3% of residual oxygen (lambda=about 1.15) is in practice the minimum residual oxygen level to which the burner can be adjusted. The excess air of a burner can also be adjusted to a lower level by a feedback of the oxygen measurement conducted from flue gases. With oxygen measurement, the combustion air or fuel of a burner is controlled by burner automation so as to maintain the oxygen content of flue gas at about 2-3%. In addition to oxygen control, it is possible to employ carbon monoxide control which adjusts the residual oxygen to a lower level until small amounts of carbon monoxide begin to appear. This adjustment may enable an achievement of the residual oxygen level close to about 1% (lambda=about 1.05).
The use of a catalytic post-combustion method as described in the above-mentioned application has been said to require that an approximately stoichiometric air-fuel ratio during the thermal combustion process be maintained consistently.
It was now unexpectedly discovered in the invention that the stoichiometric fuel and air ratio required by the discussed method is practically unreachable with existing automation and fuel supply solutions in a design, wherein the accompanying flue gas conduit or boiler is provided with a so-called three-way catalytic converter. Even with several possible burner adjustment methods available, there is none that would achieve a sufficiently low residual oxygen level required for flue gases by the method described in the discussed application.
By just regulating the rate of total fuel flow and total combustion air flow, it will be difficult to reach the required concentrations of residual oxygen prior to the catalytic zone as a result of physical, burner control technology-related, as well as hardware-related limitations.
Additionally, it was discovered, that while it was sometimes possible to attain stoichiometric fuel and air ratio in a mixer zone of a burner and thereafter a proper flue gas composition before delivering this flue gas composition into catalytical zone with above “existing technology” for example by regulating the air/fuel ratio supplied into a burner conventional technology, “it was practically impossible to maintain said proper composition of flue gases except a very short time span.